Low temperature refrigeration apparatus and process



y 1961 E. AMBLER 2,982,106

LOW TEMPERATURE REFRIGERATION APPARATUS AND PROCESS Filed July so, 1959 3 Sheets-Sheet 1 AUXILIARY STORAGE FOR e //aLI* 4 /3 MERCURY EJECTOR l9v PUMP 3:3,! LIQUID N2 BATH PUMPL DIFFUSION sPUMP 24 H51 P2 LIQUID He BATH ATO..9-0.8K Fly? 1 W I I UTILIZATION THROTTLE I I CHAMBER aol I J 60 3H0" 0.2 sx10' 10' 10 0.05 RANGE APPROX. 3 e00 3x10 0.5 1.8Xl0 10- 10 0.3 to

i 6000 3xlo' 0.36 lo- 10- 10' 5. e

. (or/00m) TEMPERATURE-PRESSURE CONDITIONS OF H6 INVENTOR Ewnesz Ambler w c M ATTORNEY May 2, 1961 E. AMBLER 2,982,106

LOW TEMPERATURE REFRIGERATION APPARATUS AND PROCESS Filed July 30, 1959 3 Sheets-Sheet 2 INVENTOR Ernesfflmb/r 5 .JK? ATTORNEY E. AMBLER May 2, 1961 LOW TEMPERATURE REFRIGERATION APPARATUS AND PROCESS Filed July 50, 1959 3- Sheets-Sheet 3 mum .545 MES:

INVENTOR Emefaz Ambler k w wkbb ATTORNEY YBY Un ts l States Par n LOW TEMPERATURE REFRIGERATION APPARATUS AND PROCESS Ernest Ambler, Bethesda, Md.,' assignor to the United 7 States of America as represented by the Secretary of Commerce Filed July so, 1959, Ser. No. 830,677

8 Claims. c1. 6 2-9) The present invention relates to a refrigeration system for the production of extremely low temperatures of the order of 1 K. and below and below and particularly contemplates a continual recycling evaporative system employing He as the refrigerant.

In refrigerant systems employing a refrigerant which evaporates under reduced pressure, a practical limit is reached for the lowest temperature which may be atrtained. This lower point can be reached only with great difiiculty because of the following limiting factors; i.e., (a) the vapor pressure of the refrigerant becomes very small, ([2) the volume rate of efflux of the refrigerant caused by evaporation due to heat leak and other effects becomes very large, (c) the flow resistance of the tube connecting the refrigerant bath to the pump causes a relais successively cooled in stages to the temperatures of liquid nitrogen and He under reduced pressure; evaporation of the then liquified He results in the attainment of temperatures below 1 K.

In its preferred form the low temperature refrigeration system of this invention employs recycling of the isotope He in a refrigeration circuit that has no moving parts. The isotope He as opposed to He, does not exhibit superfluidity and, secondly, the vapor pressure is higher for He than for He thereby permitting'He to be pumped to a lower pressure and temperature.

By recycling the refrigerant a smaller quantity of He may be used, or alternatively, a larger heat input may be tolerated with the same amount of refrigerant. Simplification of design results from the fact that there are no moving parts in the He circuit thereby lessening the possibility of loss of the rare and expensive He through leakage, and also providing a quiet and vibrationfree operation.

It is accordingly an object of this invention to provide a method and apparatus for producing low temperature refrigeration utilizing the isotope He A more specific object of the invention is to provide a system for producing low temperature refrigeration utilizing He as a refrigerant wherein there are no moving parts in the He circuit.

A further object is to provide a system for producing low temperature refrigeration wherein a continuous recycling process is utilized.

Still another object of this invention is the utilization of He under reduced pressure. 1

Other uses and advantages of the invention will come apparent upon reference to the specification and drawings in which:

2,982,106 Patented May 2, 1961 Fig. 2 is a table showing the thermodynamic conditions of the He involved in the operation of the present invention. Column (3) of the table shows the final temperature in degrees K. obtained in the practice of the invention while columns (4)(8) show the corresponding pressures in various portions of the He recycling loop of Fig. 1. The portions of Fig. l where such pressures, exist are labeled with corresponding. pressure designationsin Fig. 1.

Column (1) in Fig. 2 shows the heatcapacity of a particular degree of refrigeration obtained in accordance with the principles of this invention while column (2) in the table of Fig. 2 indicates the rate of flow involved.

Referring to the drawings, there is shown in Fig. 1 anauxiliary storage container- 10 for the gaseous He employed in connection with the invention. The actual amount of He needed for the practice of the invention may be very small, less than one liter of gas at standard temperature and pressure (0 C. and 760 mm.) being required. The gaseous He may be applied to the refrigeration system through a conduit 11 and a shut-off valve 12. The refrigeration system as diagrammatically illustrated in Fig. 1 comprises a closed recycling path including conduit 11A. Once the He is introduced into the recycling loop comprising inlet conduit 11A and outlet conduit 11B it is continuously recycled by means of diffusion pumps 18 and 19.

Conduit 11A is connected to a heat exchanger Ila-1 immersed in a liquid nitrogen bath or pre-cooler13.

When the low pressure stream of He enters bath 13, the pre-cooler markedly reduces the temperature of the He gas to the approximate temperature of the liquid nitrogen in the bath, namely 77 K. At this reduced temperature, the gas flows through a second heat exchanger 11a-2 immersed in a liquid He bath 14 at 4.2 K. The gas then passes through a third exchanger 11a-3 immersed in a liquid He bath 15 at a temperature in the order of 0.9 K. to 0.8 K. The gaseous He liquifiesin bath 15 at a pressure in the range of 3 to 6 millimeters of mercury (or more) as shown in column (8) of the table of Fig. 2 and hereinafter to be discussed in detail. A conventional oil diffusion pump 23, connecting by aconduit 24 with the He" bath 15 keeps the vapor preslivered through a throttling valve 16 which regulates the flow of the refrigerant to the lower portion of conduit 11B of the refrigeration loop wherein the refrigerant is allowed to evaporate and expand to a very low pressure (column (4), Fig. 2) in order to produce further cooling. As hereinafter to be described in connection with Fig. 3 of the drawings, the lower portion of conduit 11B makes thermal contact with the top surface of a utilization chamber 30.

Still referring to Fig. 1 of the drawings, the apparatus within the broken line portions corresponds to the portion of the refrigeration system inserted within an evacuated chamber 20, hereinafter to be discussed in detail with respect to Fig. 3 of the drawings. By means of a suitable mechanical or diffusion pump 21 attached by conduit 22 to chamber 20, the pressure within chamber 20 is reduced to a pressure less than 1 micron of mercury absolute.

For purposes of description, the apparatus for pumping the He refrigerant through the referred-to recycling loop consists of a Conventional diffusion pump 18 of 500 liter/second capacity backed with a mercury ejector pump 19 operating at a backing pressure of up to approximately mm. Hg. 7 a

The portion of the conduit in thermal contact, with utilization chamber 30 will hereafter be referred to throughout the body of the specification as the He outlet conduit 11B. The He refrigerant in conduit 11B rejoins conduit 11A downstream ofshut-off. valve 12 whence the cycle is repeated. i i

Referring again to the table of Fig. 2, the three'horizom tal rows contain data concerning the capacity of refrigeration at three levels of heat influx, ile. 60, 600, and 6,000 ergs/sec., respectively. In the chart of Fig.2, qvdesignates the heat influx or heatleak into the system in ergs/sec. dn/dt is the rate at which the gas'is circulated in mol/sec., T corresponds torthe final refrigeration temperature produced measured on the absolute temperature scale, and p p represent the pressures'in millimeters of mercury at the corresponding points indicated in the refrigeration cycle of Fig. 1. The refrigeration temperature is dependent upon the rate at which the gas is circulated, the pressure on the gas prior to expansion and the amount of heat influx into the system.

Inspection of the chart of Fig. 2 indicates that a temperature of 036 K. may be produced utilizing applicants method at a heat influx of 6,000 ergs/sec., whereas 03 K. and 02 K. may be attained at a heat influx corre-' sponding to 600 or 60 ergs/ sec.

Although lower temperatures than 0.25 K. may possibly be achieved in accordance with the principles of this invention, the use of conventional diffusion pumps limits pumping at the extremely low vapor pressures indicated by column (4) of the table of Fig. 2. By scaling up the entire apparatus it will be apparent that a much higher refrigeration capacity than that shown in Fig. 2 may obviously be achieved.

Figs. 3 and 4 show an implementation of the refrigeration system of this invention diagrammatically illustrated inFig. 1. Corresponding elements are designated by the same reference characters in Figs. 3 and 4.

Referring more particularly to Fig. 3 of the drawings, in a preferred embodiment of this invention the portion of the apparatus designated as 20 in Fig. 1 is shown in Fig. 4 as including a. cylindrical vessel of hollow form which is inserted within a conventional Dewar 25A and immersed in a liquid He bath 14 at 4.2 K. The chamber can be constructed of brass or other suitable material. In a modified version of the invention, the liquid He bath 14 may be insulated from the outside environment by inserting the vessel 20 and Dewar 25A in an additional bath 13 of liquid nitrogen contained within a second Dewar 25B.

The chamber 20 is evacuated by the previously-described diifusion pump 21 which is connected by conduit 22 to chamber 20 as shown in Fig. 4. By means of such arrangement the pressure within the chamber may be maintained at less than 1 micron of mercury absolute.

The detailed construction of chamber 20 and the apparatus carried therein is illustrated in Fig. 3. The bath is shown in Fig. 3 as comprising a cylindrical container constructed of copper or the like, into which the He comprising the coolant is introduced in a manner hereafter to be described.

The bath 15, as detailed in Fig. 3, is connected by a conduit 24 which passes through a sealed opening in wall a of chamber 20. A lateral extension 24a is provided in conduit 24 below the He liquid level in Dewar 25A. Conduit 24 connects with the referred-to oil diffusion sure'of the He 'w'ithiri bath 15. The lateral extension 24a of conduit 24 connects with an outlet 27b of a He feed valve 27.

In a manner heretofore described in connection with Fig. 1 of the drawings, gaseous He is delivered from stor' age container 10, through shut-off valve 12 and inlet conduit 11A to a heat exchanger 11a-2 which is integral with conduit 11A.as previously described in connection with Fig. 1. As illustrated in Fig. 3 of the drawings, heat exchanger 11a-2 passes through a sealed opening provided in the upper surface 20a of chamber 20 in a conventional manner. Gaseous He passes through heat exchanger Ila-2 at the approximate temperature of the He hath, 14, 42 K.

The heat exchanger Ila-2 includes a first capillary 1111-4, preferably constructed of stainless steel or the like and having an inside diameter of a few thousandths inch which helps to regulate the flow of gaseous He to heat exchanger Ila-3. Heat exchanger Ila-3 as previously described in connection with Fig. 1 traverses bath 15 through sealed openings in the upper and lower surfaces of bath 15.

As previously described, the bath 15 of liquid He is maintained at a temperature of 0.9 K.-0.8 K. by the pump 23 and causes the gaseous He in heat exchanger Ila-3 to liquify at a pressure in the order of 3 to 6 millimeters of mercury absolute.

The heat exchanger Ila-3 is connected to a second capillary Ila-5 as shown in Fig. 3 and is also suitably constructed of stainless steel or the like and has an inside diameter of a few thousandths of an inch. The second capillary 11a-5 regulates the flow of the He to the conduit 11B of the refrigeration loop. As indicated in Fig. 3, capillary Ila-5 makes thermal contact with the top surface of a utilization chamber 30. A boss 31a provides good thermal contact with the fluid in capillary Ila-5. The liquid He is evaporated and expanded to a very low pressure in the region where the capillary Ila-5 joins the return portion of the conduit to produce cooling in the manner heretofore described with reference to Figs. 1 and 2 of the drawings.

The capillary Ila-5 is connected to a return conduit 11B which passes through sealed openings in the lower and upper surfaces, respectively, of bath 15 to the upper surface 20a of container 20.

The return conduit 11B is connected to the He' pumping system heretofore described in connection with Fig. 1 of the drawings and hereafter to be more fully discussed in connection with Fig. 4.

Sample or utilization chamber 30, mounted in the lower portion of container 20 is of hollow cylindrical form and is preferably constructed of copper or the like. A vertical rod 31b is provided integral with the upper wall of chamber 30 and provides thermal contact between the wall 31 and the interior of the utilization chamber. Fig. 3 shows a convenient means for suspending a sample S by means of linen threads 33, or the like, within sample chamber 30. Chamber 30 is initially evacuated to a pressure in the order of 10- mm. mg.

The portions of the conduit identified in connection with Fig. 3 of the drawings, conduits 24, 11A, 11B, and 22, respectively, are indicated in Fig. 4. As shown in Fig. 4 of the drawings, said conduits pass through sealed openings in a horizontal copper plate 34 and conduits 24 and 11B terminate slightly above the upper surface 34a of plate 34. The upper rims of Dewars 25%25B, heretofore described in connection with Fig. 3 of the drawings, terminate in proximity to the bottom surface 34b of plate 34 as shown in Fig. 4 of the drawings.

The diffusion pumps 18 and 23 described in connection with Fig. 1 are also shown in Fig. 4 as including pumping tubes 35A and 35B. The ends of these tubes 35A and 35B are sealed to the upper surface 34a of plate 34 in a position to receive theends of conduits 24 and 11B, respectively, which pass through-plate 34 and ter x w v,

i disclosure relates; to'only minate Within tubes SSA-435B slightly above the surface 34a. Pumping tube 35A terminates in a flange 35a which connects in a conventional manner with flange 23a of oil diffusion pump 23. In a preferred embodimentof the invention pumping tubes 35A-35B preferably should be of large diameter to obtain a gain in pumping speed.

The high vacuum pumps such as 18 and 23 employed are conventional and may be of the type described in US. Patent No. 2,572,449. A copper coil 36, in which water is circulated, encircles the casing of pumps 19, 18, and 23, respectively, in series, and assists in condensation of the working fluids therein.

The upper end of pump 23 as shown in Fig. 4 is the high vacuum or intake side of the pump. The exhaust side of the pump is connected by conduit 24a-2 to a conventional fore pump 37A. Reference number 38 designates a regulating valve which is positioned within conduit 24a-2.

As described with respect to Fig. l of the drawings, pump 23 reduces the pressure within bath 15 to less than a micron of mercury absolute. Referring also to Fig. 3 of thedrawings, with piston 27c of valve 27 positioned so as to constrict outlet 27b, He pump 23 reduces the pressure within bath 15 to obtain a hard vacuum. Upon raising piston 27c, liquid He enters inlet 27a. of valve 27 and flows into the evacuated chamber 15- by way of outlet 27b, lateral extension 24a and conduit 24. After repositioning piston 270 so as to again constrict outlet' 2712, pump 23 is utilized for pumping on the He hath 15. By pumping on the He within bath 15 and thereby reducing the pressure to less than a micron of mercury absolute, the temperature of the He within bath 15 is reduced to the order of 0.90.8 K. as previously in- 'dicated.

As is further shown in Fig. 4- of thedrawings, a conventional glass mercury diffusion pump 21 is connected to chamber 20'by conduit 22 passing through a sealed opening in the upper wall 20a. Fore pump 37B connects to the exhaust side of pump 21 by conduit Me.

As heretofore described, pumping tube 35B (Fig. 4) connects with the He return conduit 113 which passes through plate 34 and extends 'a slight distance above surface 34a into tube 35B. A gate valve 35b is placed in tube 35B and controls the fiow of gaseous He in return conduit 11B. Tube 35B terminates in a flange 35c which connects with flange 18a-of the previously-identified oil diffusion pump18.

cury diffusion pump 19 which may be of the type'described in British Patents No. 700,978 and 721,667.

The exhaust end 18b of a A pump 18 is connected through aflange 19a to the merinvention and that numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is: 1. A method of refrigeration for producing temperatures substantially below 1" K. comprising the steps of cooling He to a temperature of about 0.8-0.9 K. by subjecting it to the cooling action of liquid He boiling under reduced pressure until said He is liquified and causing the liquid He to expand and evaporate.

2. A method of refrigeration for producing temperatures substantially below 1 K. comprising the steps of cyclically cooling He to a temperature of about 0.8-O.9 K. by subjecting it to the'coolingaction of liquid He boiling under reduced pressure until said He is liquified and causing the liquid He to expand and evaporate.

3. A method of refrigeration for producing temperatures substantially below 1 K. comprising the steps of continuously circulating He through a cooling bath of liquid He boiling under reduced pressure until said He is liquified and causing the liquid He to expand and evaporate.

V 4. A method of refrigeration for producing temperatures substantially below 1 K. comprising the steps of successively subjecting Heito the cooling action of liquid nitrogen, liquid He, and liquid He boiling under reliquid He to expand and evaporate.

5. A method of refrigeration for producing temperatures substantially below 1 K. comprising the steps of cyclically cooling He to a temperature of about 0.8-0.9" K. by successively subjecting said He in stages to the cooling action of liquid nitrogen, liquid He, and liquid He boiling under reduced pressure until said He is liquified and causing the liquid He to expand and evaporate;

6. A method of refrigeration for producing temperatures substantially below 1 K. comprising the steps of cyclically cooling He to a temperature of about 0.8-0.9 K. by successively subjecting said He in stages to the cooling action of liquid nitrogen, liquid He, and liquid He boiling under reduced pressure until said He is liquified at a pressure in the range of 3-6 mm. Hg. and causing the liquid He to expand and evaporate.

7. A very low temperature refrigeration system com prising a closed loop, refrigerant recycling apparatus containing He asa refrigerant, means for cooling said He Difiusion pump 18 may be 500 liter/sec. capacity backed with pump 19 operating at a backing pressure of up to thefiow of gaseous He in return conduit 118.. p

The He storage container ltl'diagrammaticallyshown to a temperature substantially below 1 K. comprising a plurality of heat exchangers connected to said closed loop apparatus for cooling said He refrigerant in stages,

said heat exchangers being arranged in series in the direction of refrigerant flow, said heat exchangers containing liquid nitrogen and liquid He respectively as a coolant,

in-Fig 1.v may preferably include; as illustratedin Fig.

4 of the drawings, a primary storage container 10' of ltl-litercapacity and an auxiliary storage container 10 of l-litercapacity. The two containers are joined by conduits 11 '11, valves ;12'-12, and conduits 41 -41 and 42, respectively, to the 'refrigeration loop comprising the conduits 11A and 11B.

Valve 43, positioned. in He supply conduit 11A, not: "jonly regulates the flow of gas. into the ,system but also may be employed to permit withdrawal of thefHe from the refrigeration loop into the storage containers 10-102:

That is, by closingavalve 43 the difiusion pumps will withdraw the He? gas fromv the recycling loop into cont ainers 10-40. thereby evacuating theHe gas withing" the system."

- It should be understood, g2 am; art-"11;. reta ding e a ejabqd m swm pumping means in'said closed ,loop apparatus for continuously. recycling said He? refrigerant through said heat exchangers for liquifying said refrigerant and means insaid loop for expanding said liquid refrigerant and heat exchangers include afirst and second heat exchanger containing liquid I-le and means for reducing the vapor pressure insaid second heat exchanger to cause boiling of the He coolant therein. v i I A "References Cited inthe fileof patent j y UNITED STATES PATENTS 

